1. Field of the Invention
The present invention relates to an improved process for preparing a water-soluble cationic bead polymer having an excellent water-solubility.
2. Description of Prior Art
Water-soluble high molecular weight polymers which are obtained by polymerising monomers such as neutralized salts or quaternary compounds of dialkylaminoalkyl (meth)acrylates have important applications, for example, as flocculating agents, soil conditioners, drainages, retention aids, sizing agents and treatment of fibers.
Recently, such polymers have been used in large amount as dehydration accelerators for surplus sludges formed in activated sewage treatment systems in feces and sewage treatment plants and chemical and food processing plants and have taken an important place among waste water treating agents.
If a water-soluble cationic polymer is used as a flocculating agent, the higher its molecular weight, the greater its effectiveness. In a method wherein a monomer such as a neutralized salt or a quarternary ammonium salt of a dialkylaminoalkyl (meth)acrylate is polymerized to produce a water-soluble polymer, it is possible to employ methods in which water-soluble polymers, for example, polyacrylamide are prepared by the usual radical mechanism as in aqueous solution polymerizations, precipitation polymerization and suspension polymerizations.
In order to obtain high molecular weight polymers in the aqueous solution polymerization, the polymerization is carried out in a monomer concentration of 10% by weight or higher and thus the products which are gummeous and contain water are formed. Since such polymers take a long time for dissolution and are also difficult to transport, they must be powdered by grinding and drying. Also, in this method the molecular weight is reduced considerably due to the difficulty of stirring the reaction mixture during polymerization for the removal of the reaction heat, resulting in non-uniform products.
Although according to the precipitation polymerization uniform and powdery polymers are easily obtained, a solvent having high polarity and being capable of dissolving neutralized salts and quarternary ammonium salts of dialkylaminoalkyl (meth)acrylates should be selected. In this case, it is difficult to obtain polymers of high molecular weight due to a significant chain transfer action of solvent during polymerization and the products obtained tend to be finely divided powders.
On the other hand, the suspension polymerization is an advantageous polymerization method for easily obtaining high molecular weight polymers in the form of powder due to a great capacity for removal of reaction heat. According to a conventional method, for example, as disclosed in British Pat. No. 841,127 or U.S. Pat. No. 3,211,708, an aqueous solution of monomer is polymerized in an inert dispersing medium using a nonionic emulsifier in the form of water-in-oil or oil-in-water type emulsion.
When this method is applied to the polymerization of monomers comprising neutralized salts and quarternary ammonium salts of dialkylaminoalkyl (meth)acrylates, it is difficult to form emulsions owing to the salting-out effect of these salts. Even if the emulsions are formed under appropriate conditions, for example, by using a very low concentration of the aqueous monomer solution, there will be a tendency to form polymers in the form of very fine powder after polymerizing and drying. These very finely powdered water-soluble polymers exhibit a very slow rate of dissolving since they bring in air bubbles upon dissolving in water. Moreover, these polymers are scattered into dust during the dissolving operation and the working atmosphere becomes polluted. In order to obtain products which are not in the form of very fine powder by the suspension polymerization, an aqueous solution of monomer should be polymerized in the state of drop dispersion by using a suitable dispersion stabilizer but not by forming an emulsion with an emulsifier.
A method for polymerizing an aqueous solution of water-soluble vinyl monomer by suspending it as droplets is disclosed in Japanese Patent Publication No. 9,656/1967. According to the method, the polymerization is carried out in a dispersing medium based on a halogenated olefin in which an oleophilic cellulose derivative is dissolved and it is necessary to adjust the specific gravity of the dispersing medium so that it is equal to or more or less higher than the specific gravity of the aqueous monomer solution by admixing a hydrocarbon. There are found in this Publication examples of polymerization in which from 30 to 40% by weight aqueous solutions of sodium acrylate and sodium acrylate-acrylamide, sodium methyacrylate-methacrylamide and sodium acrylate-vinyl pyrrolidone mixtures are used. It is impossible to polymerize these monomers at a higher concentration due to the limitation of their solubility in water. Accordingly, although the products obtained are water-containing spherical gelatinous substances free from fine polymers, it is difficult to convert them into a solid product by an industrial process.
One known method using a cellulose derivative as a dispersion stabilizer, is disclosed in Japanese Patent Publication No. 47,304/1972, wherein the polymerization is carried out by suspending in an aliphatic or aromatic nonpolar solvent a water-soluble cellulose compound, which is insoluble in the dispersing medium, and adding an aqueous solution of vinyl monomer to the suspension. This method has, however, a tendency to give very finely powdered products.